The present invention relates generally to systems and methods for controlling growth of crystals or thin films, and more particularly to system and method for controlling alloy composition during molecular beam epitaxy.
Molecular beam epitaxy (MBE) grown group III-V ternaries are known to suffer from problems associated with deep compensating centers, non-radiative traps, surface/interface imperfections, and alloy clustering. Although some evidence shows that growth at high substrate temperature can improve optical and electrical properties of such layers, high temperature growth is often avoided because of desorption effects which reduce composition and thickness control.
The invention utilizes desorption mass spectrometry (DMS) for control of alloy composition during MBE growth of group III-V structures under conditions for which desorption is significant enough to appreciably affect epitaxial layer composition. The invention may be generally applicable to any high or ultra-high vacuum deposition method, although successful demonstration of the invention was performed in an MBE growth chamber. A mass spectrometer (MS) is positioned on the center port or one of the oven ports of the source flange of a conventional MBE chamber for continuous monitoring of the desorbed fluxes of the non-incorporated fraction of incident species. Growth rate and layer composition are determined from incorporation-rates, and incorporation rates are given by the difference between incident flux and desorbed flux; the composition of the growing layer is then exactly deduced from the measured desorption rates when the MS is calibrated and the incident fluxes are known. The MS is calibrated by monitoring its signal when none of a given incident species is incorporated, i.e., under conditions for which the incident flux equals the desorbed flux. The measured desorbed flux is compared with its desired value (as by computer based control), and an appropriate error signal is generated and converted into a new setpoint for one or more growth parameters. Arbitrarily graded structures may be produced by programming the appropriate desorbed flux versus time profile.
It is a principal object of the invention to provide system and method for control of alloy composition during MBE.
It is another object of the invention to provide system and method for controlling alloy composition during MBE of layer compositions comprising group III-V elements.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.